The present invention relates to the field of magnetron sputtering for producing coatings on substrates and more specifically to planar magnetron sputtering with a rectangular cathode, which can be extended to a desired length, having a high utilization of target materials, high sputtering efficiency, and long target lifetime.
Magnetron sputtering is one of the most common techniques in production and laboratory for the deposition of thin films of various materials including conductors, semiconductors, and insulators. The magnetron sputtering is usually conducted using a so-called magnetron cathode in a vacuum chamber with the presence of one or more sputtering gases maintained under relatively low pressure. The magnetron cathode includes a cathode body and a target. The target is the material to be deposited onto the substrate, while the cathode body includes mainly permanent magnets or electromagnets, magnetically permeable materials, and cooling devices. The permanent magnets, with the aid of the magnetically permeable materials, generate a proper magnetic field above the target surface. During magnetron sputtering, a negative bias of a DC power (or an AC power) is applied to the target, establishing an electric field between the target and the grounded vacuum chamber and/or the grounded substrate. The movement of charged particles in the electric and magnetic fields is governed by the equations
d(mV)/dt=q(Vxc3x97B+E)xe2x80x83xe2x80x83(1)
and
d(r)/dt=V,xe2x80x83xe2x80x83(2)
where m is the mass, V the velocity, t time, q the charge, E the electric field, B the magnetic flux density, r the position. A properly designed magnetic field acting together with the electric field can confine high-energy electrons, which are very light, to the vicinity of the target surface and produce efficient ionization of the sputtering gas. The magnetic field in a normal magnetron cathode has little effect on the movement of the ions of the sputtering gas (usually Ar) due to their heavy mass. Therefore, the ions just follow the direction of the electric field and strike the target, causing sputtering of the target material that is subsequently deposited onto a substrate that faces the target. These characteristics lead to two distinctive features of magnetron sputtering, i.e., a relatively low gas pressure and low voltage to maintain glow discharge; and a high plasma density and few bombardments of high-energy electrons to substrate to deposit film at high rate with low substrate temperature.
The ionization distribution above the target surface determines the shape of erosion in the target and consequently the target utilization rate. The location of ionization caused by a collision between a high-energy electron and an atom of the sputtering gas is generally a random event. In the region where high-energy electrons appear more often there exists a higher probability of ionization. Most conventional magnetron sputtering cathodes have a closed and convexly arched shape of magnetic flux over the target surface that is usually flat. Since the magnetic and electric fields are not uniform, electrons in a magnetron discharge move in a complicated manner in a three dimensional space above the target surface. Their velocities change with time and position. However, as a result of the (Vxc3x97B+E) drift, electrons tend to pass the center of the arched magnetic flux, where the vertical component of the magnetic flux density is zero and the magnetic field is completely parallel to the target surface. This feature leads to a relatively dense ionization in that region and produces strong erosion of target therein in a conventional magnetron. As a result, the target is usually eroded in a narrow V-shape, giving a target utilization of only about 20% to 30%. Since most of the commonly used target materials are very expensive, such a low target utilization increases the costs of sputtering process and wastes a lot of target materials, some of which arc difficult to recycle. Therefore, a significant increase in the target utilization is always highly desired.
Over the years, numerous efforts have been made to improve target utilization in a sputtering magnetron. McLeod, in U.S. Pat. No. 3,956,093, increases the erosion area of target by using an electromagnet coil to generate a variable magnetic field above the target surface so that the center of the erosion groove shifts in an oscillatory manner [1]. A later U.S. Pat. No. 4,444,643 of Garrett similarly improves the utilization by mechanically moving the entire magnet assembly behind the target [2]. These improvements have been obtained with the expense of additional electrical and/or mechanical complexity. Class in U.S. Pat. No. 4,198,283 proposes using a target with special cross sectional shape that follows the magnetic flux line above the target surface [3]. This improves the target utilization at the cost of target machining. Actually, many ceramics targets are expensive to machine, while most standard target materials have a flat surface. Therefore, sputtering systems using a flat target with static and simple magnet assembly are still preferred in production and laboratory. Due to these facts, Morrison and Charles in U.S. Pat. Nos. 4,162,954 and 4,180,450 disclose planar magnetrons with flattened magnetic flux generated by static sources and improve target utilization [4, 5]. Welty in U.S. Pat. No. 4,892,633 further improves the utilization to xcx9c50% by setting a magnetically permeable shunt between center magnet and outer magnet and consequently changing the shape of the magnetic flux over the target surface [6]. However, the magnetic shunt meanwhile weakens the strength of the magnetic field and, therefore, limits the maximum allowable target thickness and sputtering rate. Later, Manley, in U.S. Pat. No. 5,262,028, proposes an alternative magnet assembly and achieves also high target utilization xcx9c50% [7]. In Manley""ss magnetron, relatively small magnets are used and set on a magnetically permeable plate to generate an idea shape of magnetic flux over the target surface, especially in the straight section of a rectangular cathode. Therefore, the plate actually shunts the magnetic field. This again limits the sputtering efficiency for relatively thick target, as some high-energy electrons cannot be confined effectively due to scattering by collision with sputtering gas atoms. In addition, to sputter some ceramic targets or to conduct reactive sputtering, a relatively strong magnetic field over the target surface is desired to avoid charge accumulation and consequent arcs in target surface.
As a summary, it is still highly desired to develop new sputtering magnetrons with relatively simple or static magnet assembly to achieve higher utilization of target. Also, the magnetron cathode should be able to produce a relatively strong magnetic field over the surface of a thick target, i.e. xcx9c12 mm, to realize high sputtering efficiency and long target lifetime.
As mentioned before, the movement of a charged particle in a sputtering magnetron is governed by both the magnetic field and the electric field. Therefore, three-dimensional distribution of the magnetic and the electric fields over the target surface should be taken into account in the magnetron design. Unfortunately, all previous efforts only concentrated on the design of magnetic field. The role of electric field that is actually determined by the distribution of space charges that are highly non-equilibrium in the dark space above the target surface has yet been considered. Thus the magnetron design is only qualitative or empirical since no efforts have been made to optimize the magnetic and electric fields in a combined manner to maximize the target erosion through quantitative consideration of electron trajectories, electron/Ar collisions, ionization and space charge distributions. It should be noted that several authors have reported numerical simulation of sputtering magnetron [8-11]. The work has been mainly focusing on theoretical understanding the plasma discharge with no emphasis on using the computer-aided design to optimize the performance of a sputtering magnetron.
In this invention, we present a novel rectangular sputtering magnetron designed by considering not only the magnetic field but also electron drift trajectory, electron/Ar collision, ionization distribution, and electric field distribution. Numerical calculation and Monte Carlo simulation are employed to predict the target erosion. Consequently the magnet assembly, electrode shape and/or electric field are optimized to achieve a substantial improvement of target utilization. A relatively strong magnetic field is maintained over the target surface to ensure efficient sputtering.
A novel rectangular magnetron sputtering cathode is developed through computer-aided simulation of plasma discharge to provide a high utilization of target materials. The cathode includes a cooling plate that separates the cathode into two partsxe2x80x94an upper part and a lower part. The upper part includes a center pole, two side poles, two end poles, and target. The poles are made from magnetically permeable materials with a cap higher than the target surface to provide a component of electric field parallel to the target plane. The lower part includes a magnetically permeable bottom plate, on which are a center magnet, two bottom magnets, two side magnets, two bottom poles, two sets of end magnets and end poles, and end shunts. All magnets are of permanent magnet. The poles and shunts are made from magnetically permeable materials. The magnetron is designed according to the principle that the movement of charged particles in a sputtering magnetron is governed by both magnetic field and electric field. The magnet assembly in present invention is optimized in combination with electrode configuration through quantitative simulation of electron trajectories, electron/Ar collisions, space charge distribution, and target erosion profile. This provides a much higher accuracy as compared with conventional magnetron development, which only considers magnetic field distribution in a qualitative manner. Sputtering tests with Cu target show a target utilization of 55%xcx9c65%, much higher than the target utilization, xcx9c40%, in a conventional magnetron sputtering. Along with high utilization, high sputtering efficiency is achieved due to the presence of a relatively strong magnetic field over the target surface and consequently an effective confinement of high-energy electrons during the magnetron discharge. X-ray fluorescence analysis shows that pure film can be deposited with this novel magnetron cathode.